Astronomical Coordinates & Telescopes James Graham 10/7/2009 Observing Basics

• Astronomical coordinates • The telescope • The infrared camera Astronomical Coordinates

• – Stars and other astronomical objects occupy three dimensional space – Since they are at great distances from the earth, it is convenient to describe their location as the projection of their position onto a sphere which is centered at the observer Observer’s Coordinates

• Coordinates of P relative to observer O • Altitude (a) is the angle AOP – Altitude is the angle above the horizon • Azimuth (b) is the angle NOA – Degrees E from N • Observer’s coordinates (horizon coordinates) for stars change! Sunrise, Sunset & Star Trails Celestial Sphere

• Coordinates on the celestial sphere are analogous to latitude & longitude – – Celestial Poles & Equator

• N & S celestial poles are defined by the projection of the earth’s spin axis onto the celestial sphere • Celestial equator is the projection of the earth’s equator onto the celestial sphere – Objects on the celestial equator have a declination of zero degrees – The N & S poles are declination +/- 90 degrees Defining Celestial Zero Points

• The spin axis of the earth provides a natural definition of one direction • The location of the sun, when its crosses the celestial equator in the spring defines the other – Defines RA = 0

Rising & Setting

Meridian • The observer’s is the great circle drawn through the N pole, which passes directly overhead • An star transits when it crosses the meridian • The elevation of a star is greatest when it transits Sidereal

• The sidereal time is measured by the rotation of the Earth, with respect to the stars (rather than relative to the Sun) – Local sidereal time is the right ascension of a star on the observer’s meridian – One sidereal day corresponds to the time taken for the earth to rotate once with respect to the stars and lasts approximately 23 h 56 min Angle and Right Ascension

• (HA) is the angle between an observer's meridian and the hour circle on which some celestial body lies • Expressed in , minutes & seconds, HA gives the time elapsed since a celestial body's last transit (HA > 0), or the time unit the next transit (HA < 0) • Hence: HA = LST - RA Celestial coordinates

Observer’s coordinates Why is E on the Left? The 30-inch Telescope Slit Secondary

• Cassegrain Mirror cover telescope – Two mirror telescope – Primary & secondary mirrors

Primary Two Mirror Telescope

• Concave primary • Convex secondary Telescope Design

Primary

Secondary Secondary Mirror

Mirror

Focus motor

Spiders Telescope Secondary The 30-inch Telescope

Declination axis

• Equatorial mount – Hour angle axis rotates 360 degrees in 23 hours 56 minutes

Hour angle axis The Dome Dome & Slit

Slit The Infrared Camera

Control computer

Flip mirror Cold head

Camera Observing • Turn power on • Open the dome slit (check the weather) • Open the primary mirror cover • Set the flip mirror • Set focus to nominal position • Check the pointing offsets • Select a target and point – Check telescope tracking and dome orientation • Acquire an image • Close mirror cover & close slit • Power down